Cancer ranks high in the causes of death in the world. Above all, colorectal cancer is a disease being at a higher position in the mortality of cancer. In Japan, the number of colorectal cancer patients has been suddenly increasing in recent years, and about 60,000 patients suffer from colorectal cancer every year. In the number of deaths classified by organ system, colorectal cancer ranks third after gastric cancer and lung cancer. Colorectal cancer has a five-year survival rate of about 90% or more when remaining only in the colon or rectum, and is therefore known as a cancer for which early diagnosis leads to a higher healing rate. In spite of this fact, colorectal cancer is a high-mortality cancer. This is because colorectal cancer not only has high morbidity, but also shows a sudden increase in mortality with the progression of cancer, i.e., its five-year survival rate is reduced to 70% upon metastasis to lymph nodes and reduced to 25% or less upon distant metastasis to the lung or liver. For treatment of such colorectal cancer, surgical treatment and chemotherapy are commonly used, while attempts have also been made to search for cancer-specific new therapies since the recent appearance of molecular targeted drugs.
As antibody drugs approved in Japan for use as molecular targeted drugs for colorectal cancer, Avastin and Erbitux are known. They are antibody drugs targeted at growth factors such as vascular endothelial growth factor (VEGF) and epidermal growth factor (EGF). Avastin is approved for use in progressive and recurrent colorectal cancer for which curative resection is impossible. Avastin works by a mechanism of action which involves binding to VEGF to prevent its binding to VEGF receptors, thereby inhibiting vascularization and blocking nutrition to tumor tissues.
Erbitux is intended to stop the proliferation of cancer cells through binding to EGF receptors (EGFR) and thereby inhibiting EGF-mediated cell proliferation signals. Moreover, another mechanism of action also appears to work, i.e., antibody biding to the surface of cancer cells will cause antibody-dependent cellular cytotoxicity (ADCC) mediated by natural killer cells (NK cells) and/or macrophages, etc., whereby the cancer cells will be killed.
When compared to normal cells, cancer cells are characterized by having high proliferative ability, having no limit on the number of cell divisions, and causing invasion and/or metastasis to surrounding tissues. Recent studies have indicated that some limited number, but not all, of the cancer cells in cancer tissue have such properties. Namely, these limited number of cancer cells have self-replication ability (i.e., the ability to produce completely the same cells as themselves) and pluripotency (i.e., the ability to differentiate into many different cell types), which are characteristics common to stem cells including embryonic stem cells and somatic stem cells. Due to these characteristics, cancer cells would act not only to maintain the same cells as themselves in cancer tissue through self-replication, but also to generate the great majority of surrounding cancer cells as a result of differentiation. Such a limited number of cancer cells are called cancer stem cells, and there has been proposed a hypothesis that cancer occurs and progresses from these stem cell-like cells (cancer stem cell hypothesis).
Currently known cancer stem cell markers include molecular markers such as CD133, CD24, CD44 and so on (Non-patent Document 1: GASTROENTEROLOGY, 138, 2151-2162, 2010). However, antibodies against these markers bind to only some cancer stem cells and are therefore regarded as having no efficacy as therapeutic agents. On the other hand, LGR5 has a mechanism to activate Wnt/β-catenin signals through interaction with R-spondin and is therefore suggested to have the potential to be used as a cancer stem cell marker (Patent Document 1: JP 2010-532169 A).
Cancer stem cells are considered to be a major factor for cancer recurrence and/or metastasis, and the importance of targeting cancer stem cells in cancer treatment has been pointed out. However, cancer stem cells constitute only a few percentages in tumor tissue, and hence therapeutic agents designed to target only cancer stem cells would not be able to kill cancer cells in general. Namely, for cancer medication, it is an important problem to develop a new therapy targeting a marker which is expressed at an extremely higher level in cancer stem cells than in normal tissues and is also expressed in common cancer cells.
Molecular targeted drugs, typified by antibodies, are superior agents in terms of killing cancer cells upon specifically recognizing cancer. However, molecular targeted drugs will cause serious side effects in some cases when antibodies also bind to normal cells. For example, Herceptin, which is a therapeutic agent for breast cancer, may cause not only headache, asthenia, nausea and vomiting, but also interstitial pneumonia, bone marrow inhibition, hepatic disorders, renal disorders and cerebrovascular disorders. Moreover, in tissue staining, Herceptin is also known to strongly react with normal cardiomyocytes to thereby cause severe cardiac disorders (Non-patent Document 2: British Journal of Cancer, 94, 1016-1020, 2006). Further, Herceptin is an antibody drug targeted at Her2, and hence there remains a problem in that Herceptin is effective only for patients who express Her2.
In the case of Avastin, which is a therapeutic agent for colorectal cancer, its side effects include hemorrhage, thrombosis, gastrointestinal perforation, delayed wound healing, increased blood pressure and so on, among which thrombosis and gastrointestinal perforation are fatal side effects (Non-patent Document 3: Cancer Research, 57, 4593-4599, 1997). Side effects known for Erbitux include skin disorders and so on, which are not fatal but cause itching and white pustules, resulting in mental and physical burdens on patients (Non-patent Document 4: Journal of Clinical Oncology, 22, 1201-1208, 2004). Moreover, Erbitux also has a problem in that it has no effect on canceration caused by a change in signals downstream of EGFR (e.g., K-ras mutation).
Drug delivery systems are also widely known, which are designed to transport a drug by means of the ability of molecular targeted drugs (e.g., antibodies) to bind to the cell surface. In particular, recent efforts have been made to effectively deliver a drug to the surface of cancer cells and thereby more effectively kill the cancer cells, and attempts have been made to cause uptake of antibody molecules into cells for this purpose. Antibody molecules are not taken up into cells in normal cases because they cannot permeate through the cell membrane. However, certain types of antibodies are known to be taken up into cells via a mechanism called internalization through biding to membrane protein molecules on the cell membrane.
EGF receptors (EGFR), when bound to its ligand EGF (i.e., a molecule binding to these receptors), are transported into cells by clathrin-mediated endocytosis or the like, localized in endosomes and then degraded in lysosomes (Non-patent Document 5: Cell Division, 2, 11, 2007). During this process, certain types of antibodies are also known to be taken up into cells upon binding to EGF receptors, as in the case where the ligand binds to these receptors. By means of this mechanism, attempts have been started to develop a therapy such that a drug is linked to an antibody and the antibody is efficiently transported into cells.
Membrane proteins include not only receptors binding to their ligands, but also transport proteins (hereinafter referred to as transporters) which allow active or passive transport of low molecular compounds such as amino acids and sugars. These molecules have been considered not to cause internalization because they are responsible for allowing permeation of molecules inside and outside of cells.
WO2012/029990 (Patent Document 2) shows that SLC6A6 is expressed in colorectal cancer, and a monoclonal antibody recognizing its extracellular domain can be used to detect colorectal cancer and therefore can be used as a diagnostic agent. SLC6A6 (solute carrier family 6 (neurotransmitter transporter, taurine), member 6) is a 12-transmembrane protein consisting of 620 amino acids and has been registered at NCBI (the National Center for Biotechnology Information) under Reference Sequences [RefSeq] ID: NM_003043 and NP_003034.2 (SEQ ID NO: 1: nucleotide sequence (CDS: 296 to 2158), SEQ ID NO: 2: amino acid sequence). SLC6A6 is involved in taurine uptake into cells and transports taurine together with sodium ions and chloride ions.
Moreover, Patent Document 2 shows that the SLC6A6 gene is expressed in all 5 cases of colorectal cancer tissue, whereas gene transcription is not observed in 5 cases of normal tissue, as identified by in situ hybridization techniques. Further, it is shown that two clones (4B9b and 5H12d) of the obtained monoclonal antibodies each have an epitope between amino acid residues 145 and 213 in the SLC6A6 protein.
Antibody drugs against cancer still have problems, e.g., in that they will cause serious side effects and are effective only for limited patients. Thus, there has been a demand for new development of cancer-specific molecular targets and pharmaceutical preparations with fewer side effects.